The basic chemistry of the interaction of thrombin with substrates and inhibitors is studied. Of particular interest is the mechanism of reaction of thrombin with the plasma inhibitors, alpha-macroglobulin and antithrombin and the effect of the anticoagulant heparin on the latter reaction. The techniques employed are fluorescence and spectrophotometric methods in the steady-state or kinetic mode. Mechanistic studies employ traditional or computer assisted analysis of kinetic data and employ protein modification as a basic tool. Typical methods include use of the active site dye, proflavine, preparation of fluorescent dansyl thrombins or intrinsic fluorescence to follow reactions. The interaction of thrombin and other stimuli with blood platelets is an area of extension of the studies.